[Roo]

Even cylinder head quality aluminum isn't great for repeated screwing (quip here) because the threads can become work-hardened (brittle) and/or gall.
Porsche may have chosen stainless screws since they tend to have a smoother thread finish and don't oxidize (like steel bolts/screws).

Are you removing the exhausting manifolds often?

If space allowed, or the next time you remove the engine, you could install Heli-Coils or Perma-Coils in the heads.

Switching to studs also makes sense. Take your screw size and check ARP's website for studs/nuts with those pitch/thread dimensions and about 10 mm longer length.

On the chemistry side, stainless (active) and aluminum are relatively close on the Galvanic Series and anodic, so it's less likely you have a galvanic interaction occurring. If the cylinder head stainless is passive, that could be an issue. Does anyone know the material specs for the cylinder heads?
If you added water, better yet saltwater, galvanic action would be a bigger issue. Here's one of the zillion charts on the web